Effect of α-NPD film in electron transport layer on electroluminescence color change for organic light emitting devices

In order to investigate the carrier recombination zone for organic light emitting devices (OLEDs), three different organic materials were deposited in series for the emissive layer (EML) between the hole transport layer (HTL) and electron transport layer (ETL). Since the EML was supposed to emit three different colors from three separate emissive materials, the recombination zone could be investigated by observing electroluminescence (EL) spectra of the OLEDs. By introducing a hole transport material, 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) in the EML as an electron-blocking layer, the carrier recombination zone could be divided by the α-NPD layer and the EL color could be changed as the result. It has been known from this study that the electron-blocking α-NPD layer effectively limits the electron transport in the EML and divides the recombination zone to change the EL color.